<!DOCTYPE html>
<html>
 <head>
  <meta charset="UTF-8">
  <title>计算机</title>
 </head>
  <script src="jquery-3.6.0.min.js"></script>
  <script>
  $(function(){
   $("button").click(function(){
   var n1=parseInt($("#num1").val());
   var n2=parseInt($("#num2").val());
   var flag=$("#flag").val();
   if(flag=="+")
    {
    $("#num3").val(n1+n2);
    }
   else if(flag=="-")
    {
    $("#num3").val(n1-n2);
    }
   else if(flag=="*")
   {
    $("#num3").val(n1*n2);
   }
   else if(flag=="/")
   {
    $("#num3").val(n1/n2);
   }  
 });
 });
 </script> 
 <body>
 <input id="num1">
 <select id="flag">
  <option value="+">+</option>
  <option value="-">-</option>
  <option value="*">*</option>
  <option value="/">/</option>
 </select>
 <input id="num2">
 <button id="btn1">=</button>
 <input id="num3">
 </body>
</html>